1. Field of the Invention
This invention relates to a method for depositing a zinc oxide film which can be used as a transparent electroconductive film on the rear surface of a photoelectric converter as well as to an apparatus to be used with such a method.
2. Related Background Art
Various photoelectric converters utilizing thin film semiconductors made of amorphous silicon or fine crystal silicon have been developed in recent years, including those in which light passed through a semiconductor layer is reflected and absorbed again by the same semiconductor layer.
Known photoelectric converters also include those having a transparent electroconductive film typically made of metal oxide between a semiconductor layer and a reflective layer and those having an undulation structure (texture structure) on the surface of the reflective layer and/or the transparent electroconductive film for extending the light path in the reflective layer.
U.S. Pat. No. 4,419,533 discloses a technique of arranging a barrier layer typically made of zinc oxide in order to prevent the material of the reflective layer from diffusing into the semiconductor layer. U.S. Pat. No. 4,532,372 discloses a technique of forming a transparent resistive layer on the reflective layer in order to prevent any short-circuiting due to the defects in the semiconductor layer.
Japanese Patent Application Laid-Open No. 6-116722 describes a deposition method of sequentially forming a metal layer and a transparent resistive layer by sputtering, while moving a long substrate, in a continuous process as well as an apparatus that can suitably be used with such a method.
Apart from photoelectric converters, sputtering is popularly used for depositing zinc oxide film, and various techniques have been proposed to improve the light transmittance, electric conductivity and the stability of electric discharge. For example, Japanese Patent Application Laid-Open Nos. 6-2130 and 8-111123 disclose respective methods of adding another element to a zinc oxide target.
In the field of magnetron sputtering, which is not limited to deposition of zinc oxide film, Japanese Patent Application Laid-Open Nos. 5-44029 and 7-126847 describe respective methods for improving the target efficiency by defining a specific magnetic field on the surface of the target.
When using zinc oxide film for a transparent electroconductive film, the film is required to have a high light transmittance. While the electric conductivity of such a film is preferably low in many cases, the film may sometimes be required to have a low specific electric resistance. Other requirements for such a film include a high deposition rate, a stable deposition process and a high productivity.
Particularly, a transparent resistive film arranged between the reflective layer and the semiconductor layer of a photoelectric converter for the purpose of prevention of diffusion and short-circuiting is required to have a high light transmittance and a substantially high specific electric resistance. Importantly, it is also required to have a certain thickness and has to be steadily prepared at low cost for a long period.
To date, the specific electric resistance of a sputtering target made of highly pure zinc oxide is relatively high and hence gives rise to abnormal discharges involving arc discharges when fed with large electric power in a known DC sputtering process so that it can not give stable electric discharges. Generally speaking, the specific electric resistance of a target of highly pure zinc oxide is about 0.01 .OMEGA..multidot.cm at the lowest in a hot press process or in a cold press process selectively using a binder.
In recent years, various proposals have been made to realize a stable electric discharge by adding aluminum, silicon, boron or gallium to the zinc oxide of the target in order to reduce the electric resistance of the target. However, such additives can reduce the light transmittance to such an extent that the photoelectric converter comprising the zinc oxide is no longer feasible because a high light transmittance is vital to the photoelectric converter.
Thus, to date, the target is forced to have a relatively low specific electric resistance and to be fed with DC power at a relatively low rate at the cost of a low deposition rate, a poor productivity and a large deposition system.